1. Field of the Invention
This invention relates to conductive and weldable anti-corrosion compositions for coating metal surfaces and a process for coating metal surfaces with electrically conductive organic coatings.
In the metal-processing industry, in particular when constructing vehicles, the metal constituents for the product have to be protected against corrosion. According to the traditional prior art, the sheet metal is first coated with anti-corrosion oils in the rolling mill and optionally coated with drawing compounds prior to forming and punching. In the vehicle construction sector, appropriately shaped sheet metal parts are cut out for vehicle bodies or body parts and shaped using such drawing compounds or oils in a deep-drawing process, then they are assembled, generally by means of welding and/or flanging and/or bonding, and finally cleansed in a costly procedure. Anti-corrosion surface treatments, such as phosphatising and/or chromatising, then follow, whereupon a first lacquer layer is applied to the structural parts using electrodeposition. In general, this first electrodeposition process, in particular in the case of car bodies, is followed by the application of several more layers of lacquer.
In the metal processing industry, such as in the vehicle and domestic appliance construction sectors, in order to simplify the process, there is a requirement to reduce the cost of chemical anti-corrosion treatment. This may be achieved by providing the raw material in the form of metal sheets or metal strips which have already been provided with an anti-corrosion layer.
There is, therefore, a need to find simpler methods of production in which pre-coated sheets may be welded and then lacquered in an electrodeposition process in the well-proven manner. Thus, there are a number of processes in which, following phosphatising and/or chromatising in a so-called coil-coating process, an organic coating, which may conduct electricity to a greater or lesser extent, is applied. These organic coatings should as a rule be made up in such a way that they have sufficient electrical conductivity not to be impaired by the typical welding processes used in the car industry, for example electrical spot-welding. In addition, these coatings should be overpaintable with conventional electrodeposition lacquers.
In particular in the car industry, in addition to normal steel sheeting, steel sheeting which has been galvanised and/or alloy galvanised in a variety of processes, has been used to an increasing extent recently.
2. Description of the Related Art
The coating of steel sheeting with organic coatings which are weldable and which are applied directly in the rolling mill by the so-called coil-coating process is known in principle.
Thus, DE-C-3412234 describes a conductive and weldable anti-corrosion primer for electrolytically thin-layer galvanised, phosphatised or chromatised and drawable steel sheeting. This anti-corrosion primer consists of a mixture of more than 60% zinc, aluminum, graphite and/or molybdenum disulfide and also another anti-corrosion pigment and 33 to 35% of an organic binder and about 2% of a dispersion agent or catalyst. Polyester resins and/or epoxide resins and derivatives thereof are proposed as organic binders. It is assumed that this technology is the basis of the coating agent known in the industry by the name xe2x80x9cBonazinc 2000xe2x80x9d. Although this process provides some advantages as compared with the procedure described above (temporary corrosion protection with anti-corrosion oils followed by subsequent degreasing after assembling the metallic components), the process described in DE-C-3412234 still requires much improvement:
The coating is not sufficiently spot-weldable.
Adhesion of the lacquer to the pretreated substrates, preferably galvanised steels, is not always adequate, in particular when more severe forming of the sheeting is performed in car presses.
In accordance with the disclosure in DE-C-3412234, the organic binder may consist of polyester resins and/or epoxide resins and derivatives thereof. Specific examples mentioned are an epoxide/phenyl precondensate, an epoxy ester and linear oil-free mixed polyesters based on terephthalic acid.
EP-A-573015 describes an organic-coated steel composite sheet, one or both surfaces of which are coated with zinc or a zinc alloy, which is provided with a chromate film and an organic coating applied thereto which has a layer thickness of 0.1 to 5 xcexcm. The organic coating is formed from a primer composition which consists of an organic solvent, an epoxide resin having a molecular weight between 500 and 10,000, an aromatic polyamine and a phenol or cresol compound as accelerator. Furthermore, the primer composition contains a polyisocyanate and colloidal silica. In accordance with the disclosure in this document, the organic coating is preferably applied so that the dry film layer thickness is 0.6 to 1.6 xcexcm, since layers thinner than 0.1 xcexcm are too thin to provide corrosion protection. Layers thicker than 5 xcexcm, however, impair weldability. In an analogous manner, DE-A-3640662 describes a surface-treated steel sheet consisting of a zinc-coated or zinc alloy-coated steel sheet, a chromate film produced on the surface of the steel sheet and a layer of a resin composition produced on the chromate film. This resin composition consists of a basic resin, which is prepared by reacting an epoxide resin with amines, and a polyisocyanate compound. This film should also be used with a dry film thickness of less than about 3.5 xcexcm, because thicker layers greatly reduce the weldability.
EP-A-380 024 describes organic coating materials based on a bisphenolA- type epoxide resin having a molecular weight between 300 and 100,000 and also a polyisocyanate or blocked polyisocyanate, pyrogenic silica and at least one organic coloring pigment. A chromate-containing pretreatment with a high application of Cr is also required in this process. The organic layer should be not thicker than 2 xcexcm since sheets with thicker organic layers do not enable satisfactory spot-welding and there is a negative effect on the properties of the electrodeposition lacquer applied to the organic coating.
WO 99/24515 discloses a conductive and weldable anti-corrosion composition for coating metal surfaces, characterised in that it contains:
(a) 10 to 40 wt. % of an organic binder containing:
(aa) at least one epoxide resin
(ab) at least one hardener selected from guanidine, substituted guanidines, substituted ureas, cyclic tertiary amines and mixtures thereof
(ac) at least one blocked polyurethane resin
(b) 0 to 15 wt. % of an anti-corrosion pigment based on silicate
(c) 40 to 70 wt. % of powdered zinc, aluminium, graphite and/or molybdenum sulfide, carbon black, iron phosphide
(d) 0 to 30 wt. % of a solvent.
There is a need to provide improved coating compositions which satisfy the requirements of the car industry in all respects. In comparison to the prior art, the following properties of organic coating compositions suitable for the coil-coating process should be improved:
a clear reduction in white rust on galvanised steel sheeting in the spray test according to DIN 50021, i.e. better corrosion protection,
an improvement in adhesion of the organic coating to the metallic substrate in accordance with an evaluation by the T-bend test (ECCA standard) and impact test (ECCA standard),
ability to form an external skin (i.e. capacity for use as external sheet in a car body)
adequate corrosion protection, even with low rates of Cr, preferably also with Cr-free pretreatment processes,
the currently still conventional cavity sealing with wax or wax-containing products may become unnecessary due to the improved corrosion protection
adequate suitability for typical car welding processes.
The present invention provides a conductive and weldable anti-corrosion composition for coating metal surfaces, characterised in that it contains, based on the total composition:
(a) 5 to 40 wt. % of an organic binder containing:
(aa) at least one epoxide resin
(ab) at least one hardener selected from cyanoguanidine, benzoguanamine and plasticised urea resin
(ac) at least one amine adduct selected from polyoxyalkylenetriamine and epoxide resin/amine adducts
(b) 0 to 15 wt. % of an anti-corrosion pigment
(c) 40 to 70 wt. % of a conductive pigment selected from powdered zinc, aluminum, graphite, molybdenum sulfide, carbon black and iron phosphide
(d) 0 to 45 wt. % of a solvent,
and, if required, up to 50 wt. % of other active or auxiliary substances, wherein the proportions of the components total 100%.